1. Field of the Invention
The invention relates to a speed control circuit and, more particularly, to a speed control circuit for enabling a fan motor to have various speeds at any selected voltage.
2. Description of the Related Art
Referring to FIG. 1A, a conventional cooling fan 100 and a driving circuit 104 is connected to a direct current (DC) motor 102. The voltage of the driving circuit 104 is provided by a voltage source 106, and the driving circuit 104 feeds a signal, often a current signal, to the DC motor 102 to activate it. Typically, the speeds of the cooling fan 100 and the motor 102 are equal because the cooling fan 100 is linked with the motor 102.
In a conventional manner, the fan speed is fixed at any selected voltage, and thus the coil winding of the DC motor 102 must be changed to alter the fan speed at the voltage.
For the same coil winding, the relation between the motor speed and input voltage may be represented as a characteristic curve such as a line shown in FIG. 1B. Referring to FIG. 1B, the motor speed is in proportion to the input voltage, and thus when the DC motor 102 receives a input voltage of 3 volts, the speed of the motor 102 is 2500 rpm and 5 volts for 4000 rpm. Hence, for instance, if a motor speed of 3000 rpm is required under an input voltage of 3 volts, it is impossible to achieve through the conventional DC motor 102 without changing the coil winding.